Crude oil offshore storage vessel

ABSTRACT

A self-ballasting offshore oil storage tank comprises a vertical enclosure having a middle section with a cross-sectional area smaller than those of upper and lower sections joined thereto. The configuration of the upper and lower sections are made such that, with the middle section located at the water surface, oil stored within the vessel automatically apportions itself between the upper and lower sections in such a way that the weight of the oil stored in the upper section balances the buoyant effect of the oil stored in lower section.

United States Patent 91 Tam 1 CRUDE OIL OFFSHORE STORAGE VESSEL [75]Inventor: William A. Tam, Warrenville, 111.

[73] Assignee: Chicago Bridge & Iron Company, Oak Brook, Ill.

[22] Filed: Jan. 28, 1974 [21] Appl. No.: 436,905

[52] U.S. Cl 61/46.5; 114/.5 T; 220/85 A [51] Int. Cl. B65G 5/00; B65D89/10 [58] Field of Search 61/46.5, 46; 114/.5 T; 220/85 A [56]References Cited UNITED STATES PATENTS 3,552,131 1/1971 Mott et al.61/46 3,599,590 8/1971 Rego 114/.5 T

3,798,919 3/1974 Hershner, Sr 61/46.5 X

[ June 17, 1975 Primary Examiner 1ac0b Shapiro Attorney, Agent, orFirmMerriam, Marshall, Shapiro & Klose [5 7 ABSTRACT A self-ballastingoffshore oil storage tank comprises a vertical enclosure having a middlesection with a cross-sectional area smaller than those of upper andlower sections joined thereto. The configuration of the upper and lowersections are made such that, with the middle section located at thewater surface, oil stored within the vessel automatically apportionsitself between the upper and lower sections in such a way that theweight of the oil stored in the upper section balances the buoyanteffect of the oil stored in lower section.

10 Claims, 6 Drawing Figures Ila SHEET PATENTEDJUN 17 ms FIGJ FIG. 2

PATENTEDJUN 17 ms SHEET FIG. 4

CRUDE OIL OFFSHORE STORAGE VESSEL This invention relates to a novelself-ballasting offshore oil storage tank, which automaticallycompensates for the buoyant effect of the oil contained therein,regardless of the quantity thereof.

The use of offshore storage tanks for the storage of water-immiscibleliquids such as crude oil has become common. Most of these offshoretanks operate on the water-displacement principle, in accordance withwhich water is taken into or displaced from the storage tank as thevolume of stored oil varies. The design and operation of such tanks iscomplicated by the fact that the stored liquid, being lighter thanwater, exerts a substantial buoyant effect which in the case ofa largevessel can be of the order of millions of pounds. In order to maintain aconstant draft in the case of conventional floating storage tanks,extensive mechanical pumping systems are used to compensate forvariations in the buoyant effect of the stored oil. The operation andmaintenance of such pumping systems present substan tial operatingproblems. Not only are the systems complicated and therefore expensivebut there also exists the possibility that a hazardous condition will becreated through an operational error or malfunction.

In the case of storage tanks which are fixed to the bottom of the bodyof water in which they are situated, an anchoring system, typicallyincluding piles and/r ballast weights, must be used to accommodate theenormous upward buoyant effect caused by the stored liquid. Since thedesign must be based on the extreme case of the storage tank full ofoil, the anchoring system is excessively large for periods of operationsin which the tank is less than completely filled.

These difficulties are eliminated by the present invention, withsubstantial savings in cost and improved safety. In accordance with theinvention, there is provided an offshore storage tank which isautomatically self-ballasting. The tank comprises a vertically elongatedenclosure which is open at its bottom for ingress and egress of water asthe volume of stored liquid varies, in accordance with conventionalpractice. The enclosure has an upper section which extends above theliquid level of the water body and a lower section which is alwayssubmerged. The enclosure further has a crosssection of minimum area at alocation between the upper and lower sections. In use, the tank is sopositioned that its minimum cross-section is located at the level of thesurface of the body of water in which the enclosure is situated. Bysuitably proportioning the upper and lower sections of the enclosure,the oil stored in the vessel automatically distributes itself betweenthe upper and lower sections in such a way that the buoyant force due tothe stored oil below the water surface is balanced by the weight of theoil in the upper section above the water line. The self-ballastingfeature of the invention assures that a floating storage tank made inaccordance therewith will always maintain the same draft without thenecessity for a mechanical pumping system, regardless of the quantity ofstored liquid, and that the force which must be resisted by thesupporting structure of a fixed facility remains is greatly reducedcompared to conventional fixed storage tanks of the same capacity.

The invention will be better understood from the following detaileddescription thereof, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 shows a portion of one embodiment of the invention in thevicinity of the water level;

FIG. 2 shows a different embodiment of the invention, of the floatingtype;

FIG. 3 shows another embodiment of the invention which is fixed on thefloor of the body of water;

FIG. 4 shows another embodiment of the fixed type, having a differentconfiguration in the vicinity of the water level;

FIG. 5 shows a composite structure consisting of several embodimentssimilar to that of FIG. 3 interconnected to form a large capacitystorage unit; and

FIG. 6 represents still another embodiment of the invention of the fixedtype.

Referring to FIG. 1, there is depicted a portion of one embodiment ofthe invention, in the vicinity of the level of the water in which theenclosure is situated. As shown, the enclosure 10 comprises acylindrical middle section 13, an upper section 11 having a cylindricalportion 11a and a curved transition portion 11b joining section 11a tothe top of section 13, a lower section 12 having a cylindrical portion12a (equal in diameter to 11a) and a curved transition portion 12bjoining cylindrical portion 12a to the bottom of section 13. The line Lrepresents the surface level of the water 14 in which the enclosure issituated. The middle section 13 of the vessel is full ofa charge of oil15, the top surface 16 of which extends a distance Y above L and lowersurface 17 of which extends a distance X below L. In order to satisfythe equilibrium pressure condition which exist and in order to balancethe forces of buoyancy and gravity, the distance Y and X are related bythe formula where d is the density of the water and d is the density ofthe oil. Under the condition depicted, the net buoyant effect of theportion of the oil below the water surface, which extends to the bottomof middle section 13, is exactly balanced by the weight of the column ofoil in the portion of section 13 which extends above the surface of thewater. A

If there is now added to the interior of container 10 an additionalcharge of oil, the newly added oil will distribute itself under theforce of gravity between the upper and lower sections of the enclosure,causing the upper surface 16 of the oil to rise an additional distanceAI and the lower surface 17' of the oil to descend an additionaldistance AX below the water level L. Under the new conditions the levelof the oil above the surface of the water L, i.e., the distance (Y AI),will automatically be sufficient to achieve a pressure equilibriumthroughout the column of oil such that the pressure at the bottom of theoil phase, i.e., at a distance (X AX) below the surface of the water,will be equal to the hydrostatic water pressure at that depth. The netbuoyant effect of the submerged oil phase, however, will not necessarilyremain constant under the conditions described, unless the weight of theadded oil incorporated in the structure above the water level is equalto the additional buoyant force produced by the added oil enclosed inthe structure below the surface of the water.

In accordance with the invention, the tank is made self-ballasting bysuitably proportioning the upper and lower sections 11 and 12 of theenclosure 10. This result is achieved by making the configuration ofenclosure such that the cross-sectional area A of the upper surface ofthe oil phase contained therein is at all times equal to thecross-sectional area A of the lower surface of the oil.

In the structure depicted in FIG. 1, section 13 is cylindrical, whilesections 11 and 12 are also cylindrical above and below the transitionportions 11,, and 1212 respectively. Accordingly, when sufficient oil ischarged within the vessel so that the oil extends into the cylindricalportions of sections 11 and 12, the conditions for self-ballasting willbe present. In the transition portions between the extremities ofsection 13 and the cylindrical portions of sections 11 and 12, the shapeof the enclosure must be such that the interfacial areas at the top andbottom of the oil phase, i.e., the area of the interface between the oiland the air at the top of the oil phase, and that between the oil andthe water at the bottom of the oil phase, remain equal at all times.

For purposes of the invention it is necessary that the enclosure have aconfiguration such that there it has a minimum cross-sectional area at alocation between the ends of the enclosure, the location of minimum areabeing situated to correspond with the water level of the body in whichthe enclosure is positioned. With the enclosure thus positioned, theenclosure will be selfballasting if the rate of change incross-sectional area with increasing distance above the water line is afraction, equal to d /d d of the rate of change of crosssectional areawith increasing distance below the water level. As a result of thiscondition, the change in cross sectional area above the water line perunit of vertical distance is greater than, but proportional to, thechange in cross-sectional area which occurs per unit distance below thewater line.

In a container having the configuration shown in FIG. 1, in which thecross-sectional shape is circular at all points, it will be seen thatthe vertical configuration of the vessel above the water line is similarto that below the water line, but more accentuated, a given change incross-section area occurring in less vertical distance.

A more generalized embodiment of the invention is illustrated in FIG. 6.As shown, container 60 has an upper section which tapers upwardly andinwardly, meeting at a location which has a minimum crosssectional areapositioned at the level L of the surrounding body of water. Thecross-sectional areas of the enclosure above and below the water lineare so designed that the top and bottom interfacial areas of a charge ofoil introduced to the enclosure will always be equal. Accordingly, nonet change in the buoyancy produced by the oil contained within thevessel will be produced, regardless of the quantity of oil stored.

It should be noted in connection with FIG. 6 that although for practicalreasons it will be generally found more convenient to construct anenclosure which is symmetrical about a vertical axis, the invention isnot limited to such structures. The only requirement is that theinterfacial area of the top of the oil charge be at all times equal tothe interfacial area of the bottom of the oil charge. The relativeshapes of the structures above and below the water line are unimportant.Further, although the structure of FIG. 6 illustrates an operativeembodiment of the invention, the complex curves of the upper and lowersection of the enclosure may present practical difficulties inconstruction. For this reason, it will be generally preferred to makethe enclosures of the invention with individual portions that arecylindrical, spherical or conical, rather than continuously curving, asin the case of FIG. 6. Examples of such structures are shown in FIGS. 2,3, 4, and 5.

Illustrated in FIGS. 3 and 4 are two embodiments of fixed structuresmade in accordance with the invention. In each case the enclosures (30,40) are supported on the floor (31, 41 of the body of water and areprovided with suitable ballast means (32, 42) for holding the structuresin position. In these embodiments, the middle section (33, 43) of theenclosure, in the vicinity of the water line L, is cylindrical. Joinedto the tops and bottoms of the cylindrical sections are transition portions (34, 36, 44, 46) which lead from the cylindrical middle section tocylindrical upper and lower sections (37, 38, 47, 48) of the samediameter. In FIG. 3, the transition zones (34, 36) are conical whereasin FIG. 4 the transition zones (44, 46) are more-or-less spherical. Ineach case, however, the vertical rate (measured from the water line L)of change of cross-sectional area in the top section is equal to thesame fraction (d /d -d,,) of the rate of change of cross-sectional areain the bottom section, so that the enclosures are selfballasting.

In FIG. 5, there is depicted a multiple-tank storage facility comprisinga number of individual enclosures similar to that of FIG. 3 which areinterconnected by pipes 39 to form an installation of increased storagecapacity.

FIG. 2 depicts a floating storage tank made in accordance with theinvention. As shown, the enclosure 20 comprises a middle cylindricalsection 21 and an upper section comprising a conical transition portion22 joining a cylindrical portion 23. The lower section of the enclosurealso comprises a conical transition portion 24 joining a cylindricalportion 26 of the same diameter as portion 23.

Attached to the lower section of the enclosure 20 is a ring 27 ofballast material sufficient to stabilize the enclosure in an uprightposition. In the vicinity of the water line L the middle section 21 issurrounded by a cylindrical jacket 28 joined at its upper and lowerextremities to the top and bottom sections of the enclosure to define asealed buoyancy chamber 29 which is sufficient to support the weight ofthe enclosure when it is full of water (i.e., no stored oil). Thebuoyant volume is adjusted such that the structure will float with thecentral cylindrical section 21 partly above the water level and partlybelow, the ratio of the portion of section 21 protruding above the waterlevel to that extending below the level being equal to d,,. d /d aspreviously described. With the conical transition zones 22, 24 being soproportioned that a charge of oil added to the vessel maintains upperand lower interfacial areas which are always equal, there is no netbuoyant effect produced by the contained oil, regardless of the quantitythereof. Accordingly, the tank will always float with the same draft andthe anchoring chains 31 attached to anchors 32 embedded in the floor 34of the body of water will always encounter the same magnitude of forces.

In addition to the self-ballasting feature of the invention, there areseveral advantages which stem from the requirement that thecross-sectional area is a minimum in the vicinity of the water line. Thenarrowed section at the surface of the water encounters less resistancefrom wave motion and thus lowers the forces which must be resisted bythe structure. In addition, the configuration of a floating vessel suchas that of FIG. 2, with the buoyancy chamber inside the cylinder,permits the anchor system to be so designed as to minimize the tendencyof the floating vessel to heave. Similarly, the dimensions of astructure made in accordance with the invention are well adapted for thedesign of advantageous surge and pitch properties, in accordance withknown design principals.

The foregoing detailed description has been given for clearness ofunderstanding only, and no unnecessary limitations should be understoodtherefrom, as modifications will be obvious to those skilled in the art.

What is claimed:

1. A selfballasting offshore storage tank of the water-displacement typefor the storage of a liquid lighter than and immiscible with watercomprising:

a vertically elongaged continuous enclosure open at the bottom foringress and egress of water;

means for positioning said tank at a substantially fixed level relativeto the surface ofa body of water in which said tank is situated, whensaid tank contains only water filling the lower portion of saidenclosure up to the level of the surrounding body of water;

said continuous enclosure having an upper section which extends abovethe surface of said body of water and a lower section which is alwayssubmerged;

said enclosure having a cross-sectional area which has a minimum valueat the junction between said upper and lower sections, each of saidsections having a respective maximum cross-sectional area larger thansaid minimum value, said area varying above and below said junction in amanner such that the upper and lower interfacial surfaces of a body ofsaid liquid stored in said enclosure remain substantially equalregardless of the quantity of said liquid.

2. An off-shore storage tank in accordance with claim 1 in which saidenclosure has a substantially circular cross-section throughout.

3. An off-shore storage tank in accordance with claim 2 in which saidenclosure has a vertical substantially cylindrical middle section in thevicinity of the surface of said body of water, said cylindrical sectionextending a first distance above said surface and a second distancebelow said surface, the ratio of said first distance to said seconddistance being substantially equal to d,,- d ld where d is the densityof said water and d,, is the density of said liquid.

4. An off-shore storage tank in accordance with claim 3 in which saidenclosure comprises a substantially cylindrical upper section joined tothe top of said middle section and a substantially cylindrical lowersection joined to the bottom of said middle section, said upper andlower sections having substantially the same diameter.

5. A storage tank in accordance with claim 3 in which said enclosurecomprises a first, upwardly opening substantially conical section joinedto the upper end of said cylindrical section and a second, downwardlyopening substantially conical section joined to the lower end of saidcylindrical section, the ratio of the rate of increase ofcross-sectional area in the upper conical section with increasingdistance above the surface of said body of water to the rate of increaseof cross-sectional area in the lower conical section with increasingdistance below said surface being substantially equal to (l /d d where(1,, is the density of said water and d,, is the density of said liquid.

6. An off-shore storage tank in accordance with claim 5 in which saidmeans for positioning said enclosure at said fixed level relative to thesurface of said body of water comprises a substantially cylindrical l0jacket encircling said cylindrical section and joined to the enclosureat the upper and lower ends thereof to form a sealed flotation chamber.

7. An off-shore storage tank in accordance with claim 1 in which saidmeans for positioning said tank at a constant level comprises flotationmeans attached thereto.

8. An off-shore structure in accordance with claim 1 in which said meansfor positioning said tank at a constant level comprises means connectingsaid structure to the floor of said body of water.

9. An off-shore structure in accordance with claim 8 in which the lowerend of said enclosure rests on the floor of said body of water.

10. A self-ballasting off-shore storage tank of the water-displacementtype for the storage of a liquid lighter than and immiscible with watercomprising:

a vertically elongated continuous enclosure open at the bottom, saidenclosure having a top section all of which extends above the surface ofa body of water in which the storage tank is positioned, a bottomsection which is always submerged and a partially submerged cylindricalmiddle section joining said top and bottom sections;

a cylindrical jacket encircling said middle cylindrical section andjoined to said top and bottom sections to form a sealed flotationchamber, said flotation chamber having sufficient buoyancy to float saidenclosure in said body of water at a constant level relative to thesurface thereof in a position such that a portion of said centralcylindrical section extends a first distance above the surface of saidbody and a second distance below the surface thereof, the ratio of saidfirst distance to said second distance being equal to d d,,/d where d isthe density of said water and d is the density of said liquid;

said upper section of said enclosure comprising a first, upwardlyopening substantially conical portion joined to the upper end of saidcylindrical section;

said lower section of said enclosure comprising a second, downwardlyopening substantially conical portion joined to the lower end of saidcylindrical section;

the ratio of the rate of increase of cross-sectional area in said firstconical portion with increasing distance above the surface of said bodyof water of the rate of increase of cross-sectional area in said secondconical portion with increasing distance below said surface beingsubstantially equal to d /d d where d is the density of said water and dis the density of said liquid; and

conduit means communicating from the exterior of said enclosure to theinterior thereof for supplying said liquid to and removing said liquidfrom the interior of said enclosure.

1. A self-ballasting offshore storage tank of the waterdisplacement typefor the storage of a liquid lighter than and immiscible with watercomprising: a vertically elongaged continuous enclosure open at thebottom for ingress and egress of water; means for positioning said tankat a substantially fixed level relative to the surface of a body ofwater in which said tank is situated, when said tank contains only waterfilling the lower portion of said enclosure up to the level of thesurrounding body of water; said continuous enclosure having an uppersection which extends above the surface of said body of water and alower section which is always submerged; said enclosure having across-sectional area which has a minimum value at the junction betweensaid upper and lower sections, each of said sections having a respectivemaximum crosssectional area larger than said minimum value, said areavarying above and below said junction in a manner such that the upperand lower interfacial surfaces of a body of said liquid stored in saidenclosure remain substantially equal regardless of the quantity of saidliquid.
 2. An off-shore storage tank in accordance with claim 1 in whichsaid enclosure has a substantially circular cross-section throughout. 3.An off-shore storage tank in accordance with claim 2 in which saidenclosure has a vertical substantially cylindrical middle section in thevicinity of the surface of said body of water, said cylindrical sectionextending a first distance above said surface and a second distancebelow said surface, the ratio of said first distance to said seconddistance being substantially equal to dw - do/do where dw is the densityof said water and do is the density of said liquid.
 4. An off-shorestorage tank in accordance with claim 3 in which said enclosurecomprises a substantially cylindrical upper section joined to the top ofsaid middle section and a substantially cylindrical lower section joinedto the bottom of said middle section, said upper and lower sectionshaving substantially the same diameter.
 5. A storage tank in accordancewith claim 3 in which said enclosure comprises a first, upwardly openingsubstantially conical section joined to the upper end of saidcylindrical section and a second, downwardly opening substantiallyconical section joined to the lower end of said cylindrical section, theratio of the rate of increase of cross-sectional area in the upperconical section with increasing distance above the surface of said bodyof water to the rate of increase of cross-sectional area in the lowerconical section with increasing distance below said surface beingsubstantially equal to do/dw - do, where dw is the density of said waterand do is the density of said liquid.
 6. An off-shore storage tank inaccordance with claim 5 in which said means for positioning saidenclosure at said fixed level relative to the surface of said body ofwater comprises a substantially cylindrical jacket encircling saidcylindrical section and joined to the enclosure at the upper and lowerends thereof to form a sealed flotation chamber.
 7. An off-shore storagetank in accordance with claim 1 in which said means for positioning saidtank at a constant level comprises flotation means attached thereto. 8.An off-shore structure in accordance with claim 1 in which said meansfor positioning said tank at a constant level comprises means connectingsaid structure to the floor of said body of water.
 9. An off-shorestructure in accordance with claim 8 in which the lower end of saidenclosure rests on the floor of said body of water.
 10. Aself-ballasting off-shore storage tank of the water-displacement typefor the storage of a liquid lighter than and immiscible with watercomprising: a vertically elongated continuous enclosure open at thebottom, said enclosure having a top section all of which extends abovethe surface of a body of water in which the storage tank is positioned,a bottom section which is always submerged and a partially submergedcylindrical middle section joining said top and bottom sections; acylindrical jacket encircling said middle cylindrical section and joinedto said top and bottom sections to form a sealed flotation chamber, saidflotation chamber having sufficient buoyancy to float said enclosure insaid body of water at a constant level relative to the surface thereofin a position such that a portion of said central cylindrical sectionextends a first distance above the surface of said body and a seconddistance below the surface thereof, the ratio of said first distance tosaid second distance being equal to dw - do/do where dw is the densityof said water and do is the density of said liquid; said upper sectionof said enclosure comprising a first, upwardly opening substantiallyconical portion joined to the upper end of said cylindrical section;said lower section of said enclosure comprising a second, downwardlyopening substantially conical portion joined to the lower end of saidcylindrical section; the ratio of the rate of increase ofcross-sectional area in said first conical portion with increasingdistance above the surface of said body of water of the rate of increaseof cross-sectional area in said second conical portion with increasingdistance below said surface being substantially equal to do/dw - do,where dw is the density of said water and do is the density of saidliquid; and conduit means communicating from the exterior of saidenclosure to the interior thereof for supplying said liquid to andremoving said liquid from the interior of said enclosure.